1. Field of Invention
The present invention relates to the art of verifying and testing the calibration of test sensors, and more particularly apparatuses for comparing the output of surface detectors with known movements, and to a method for determining the status of calibration of object detectors in comparison with known object movements.
2. Description of the Prior Art
In the past, apparatuses for measuring accelerations and displacements caused by surface variations on the surface of a hard disk, herein sometimes designated as "hard disk surface testers" or "hard disk substrate testers," have been provided having read-out or pick-up heads or sensors borne over the surface of the hard disk having information designed for either a magnetic or optical reading therefrom. Hard disks conventionally have been made of aluminum, glass or plastic material and, while every effort is made to make the surface of the information bearing hard disk as smooth as possible, variations in the axial direction occur in that surface. Hard disks on which such information has been stored frequently are circular in shape, designed to be rotated about a center axis on a turnable, capstan or spindle, and the plane of the surface is generally perpendicular to that axis. Some information bearing surfaces may not be circular, but in all events hereinafter in this specification, the "axial direction" is used to mean the direction perpendicular to the plane of the surface of the information bearing medium.
Optically and magnetically encoded information recorded on hard disks is retrieved by pick-up heads. One frequently used method of retrieving such information is to mount such a pick-up head on an airborne slider which maintains the pick-up head and the slider in non-contact relationship with the disk so as to eliminate wear frequently found by contact-type pick-up head arrangements. The slider is airbone above the surface of the hard disk at a distance determined usually by the air currents naturally developed by virtue of the speed at which the disk is rotated about its axis. Such a distance or clearance in the axial direction above the surface of the disk is approximately two one-hundred-thousandths of an inch or less (0.0002"). The air pressure developed between the slider and the disk is normally such as to maintain the slider constant at this distance from the surface of the disk if the disk is smooth. Even when the disk's surface is characterized by long and rolling hills and valleys, the air pressure can keep the slider very close to a constant axial distance from the surface. There will, of course, be accelerations in the slider as it approaches a hill or a valley where the effects of inertial vectors of the velocity and of the change in velocity of the slider cause it to change slightly upward, downward or perhaps in other directions in its distance or displacement from the disk surface and information tracks on that surface. Hereinafter in this specification, the term "acceleration," unless specified otherwise, will be used to denote these effects in the axial direction. It is understood that the slider may be subject to vector components of acceleration in other directions, and that total acceleration is the result of energy of an absolute quantity regardless of relative direction or orientation.
Very frequently mass produced hard disks can have predictable hills and valleys which cause accelerations to the airborne slider, and will create a non-constant distance between the slider and the surface of the hard disk with only slight accelerations and displacement variations of the slider. Such accelerations and displacement variations can, however, go beyond tolerable limits, beyond which the optical or magnetic pick-up head or sensor is unable to reliably read data from that surface. Such reliability can be destroyed even by closely spaced waves or ripples in that surface. Such disturbances can cause great accelerations to the sensor, sometimes measured in the hundreds or thousands of inches per second per second. These disturbances and like physical defects on the surface of the disk are generally called "axial runout" effects and the concomitant acceleration experienced by the slider is denoted as the "runout velocity acceleration" effect or "RVA", which is a common reference term used in the industry for all of these resultant disturbances on the slider.
It is important for purchasers of hard disks to known within reasonably close tolerances the topographical characteristics and features of the hard disk surface. In high speed reading and random access data and memory implanting and retrieving, the airborne slider is designed to ride very close to the hard disk surface. Moreover, the more dense the information channels or tracks on the hard disks are, the closer the slider must be to the hard disk surface for accurate and isolated information implantation and retrieval. The closer that the slider rides to the surface, however, the more profound and consequential are the effects of surface variations on the speed and clearance of the slider relative to that surface. For the most part, these effects cause accelerations and energy transfers (RVA) to the slider. Such accelerations which act upon the slider and the pick-up or sensing head will, if they are too great, cause the slider to deviate away from the desired track or path of movement so that reliable reading or data encoding cannot be accomplished. Such accelerations may even cause the slider to fail catastrophically such as in an actual contact between the slider and the hard disk's surface. Therefore, if the intolerable topographical characteristics (RVA) of the hard disk surface are known to a prospective purchaser, the purchaser will be able to determine accurately and precisely how far above the hard disk's surface the airborne slider can be maintained without accelerations that will cause failure of the slider to maneuver efficiently, speedily and accurately over the hard disk's surface.
Disk rotation speeds and the axial displacement or clearance of the slider from the disk's surface are parameters which are normally predetermined by externally imposed specifications and operating requirements. If the effects of the topographical surface characteristics, particularly the RVA, of the hard disk can be accurately predicted or tested with reliable test results, the suitability of the hard disk to a particular job application can be determined. It has long been desired to have disk surface or substrate testers that will accurately give quantitative determinations of the variations or displacements in such a hard disk substrate or surface, and to give quantitatively accurate predictions of the effects of the topographical characteristics on the performance of a slider on that surface.
Hard disk surface testers have been provided before, and are offered by a number of manufacturers. Such testers run a probe over the disk surface in the form of a non-contact, optical, magnetic or capacitive sensor, one of the more frequently seen having a 0.066 inch diameter capacitive plate with a stand-off distance of 0.01125 inches from the disk surface. In effect, a capacitive plate forms the sensing capacitor probe which, with the disk surface itself, forms the capacitor whose capacitance is measured in an electric circuit. At the desired spin or disk rotation speed, the sensor is made to traverse radially to the radial center of the disk while the tracks or grooves of the disk are spun under it. Such disk testers can accurately reflect variations or displacement of the disk's surface and its topography in relative terms, and can give output read-outs in relation to the correlative starting value of the sensor.
Thus, it might be possible with testers now available to locate the positions of greatest run out, velocity and acceleration and to have a relative measurement of those parameters so long as a single disk surface tester is used. In many circumstances, serveral disk surface testers are employed, or attempts are made to compare in absolute, quantitative terms the results of one disk surface tester with the results of another. Presently there exists, however, no simple and expedient apparatus or method for correlating the read-outs and dynamic response curves of such testers one with another. It is stil sought but not provided to have a simple yet reliable apparatus and method for correlating the read-outs and dynamic response curves of several and indeed all disk surface or substrate testers.
The disk surface axial displacement, from which axial acceleration can be computed and determined, appears as an aberrant curve on, for example, an oscilloscope screen, and some relative measurement can be made of the aberrant cycle against graph lines on the oscilloscope screen. Such measurements, however, are in relation to the adjustment of the oscilloscope output and, therefore, there has been no way of correlating the dynamic waveform caused by such a particular surface displacement acceleration with an absolute, quantitative and determinable number that would represent the physical distance of axial displacement change caused by such RVA. Heretofore, there has not been provided, but has long been sought an accurate, simple and reliable method and apparatus for determining whether or not such hard disk substrate testers have output signals or information which are quantitatively accurate, or how far off quantitative accuracy such output information is.